Processing of the precipitation hardening nickel-base superalloys

ABSTRACT

AFTER WORKING AND PRIOR TO AGING, THE PRECIPITATION HARDENING NICKEL-BASE SUPERALLOYS ARE SUBJECTED TO A DUPLEX HEAT TREATMENT COMPRISING A FIRST HEAT TREATMENT AT A TEMPERATURE OF 25* -100*F. BELOW THE SECONDARY PHASE SOLVUS AND A SECOND HEAT TREATMENT IN THE TEMPERATURE RANGE OF 0-25*F. BELOW THE SECONDARY PHASE SOLVUS.

United States Patent Oifice 3,677,830. Patented July 18, 1972 3,677,830PROCESSING OF THE PRECIPITATION HARDEN- IN G NICKEL-BASE SUPERALLOYSArthur R. Cox, Lake Park, and Marvin M. Allen, North Palm Beach, Fla.,assignors to United Aircraft Corporation, East Hartford, Conn. NoDrawing. Filed Feb. 26, 1970, Ser. No. 14,678 Int. Cl. C22f 1/18 US. Cl.14812.7 3 Claims ABSTRACT OF THE DISCLOSURE After working and prior toaging, the precipitation hardening nickel-base superalloys are subjectedto a duplex heat treatment comprising a first heat treatment at atemperature of 25 -l00 F. below the secondary phase solvus and a secondheat treatment in the temperature range of 25 F. below the secondaryphase solvus.

BACKGROUND OF THE 'INVENTION The present invention relates in general tothe processing of the worked, precipitation hardened nickel-basesuperalloys.

The typical nickel-base superalloy is essentially a nickelchrorniumsolid solution (7 phase) hardened by the additions of elements such asaluminum and titanium to precipitate a secondary phase (7' phase),usually represented by the formula Ni '(Al,Ti). These alloys alsofrequently contain cobalt to raise the solvus temperature of the 7'phase, refractory metal additions for solution strengthening, andcarbon, boron and zirconium to promote ductility and fabricability.

Representative of alloys of this general nature are those identified inthe industry as follows:

Nominal composition 4.5% A1, 5.3% Mo, .07% C., 0.3 B, bal. Ni. Waspaloy19.5% Cr, 13.5% C0, 3% Ti, 1.4% A1, 4% Mo, .08% C, .005% B, 08% Zr, bal.Ni.

These alloys may be fabricated by direct upset forging at or near thesecondary phase solvus followed by hammer forging below the solvustemperature. After working it has been the previous practice to heattreat as close to the secondary solvus as practical followed bysubsequent aging.

Unfortunately, with this processing there has typically occurred bothextensive segregation of the secondary phase and non-uniformrecrystallization of the polycrystalline microstructure, both of theseconditions resulting in poor repeatability of mechanical properties frompart to part, especially in terms of yield and creep strength, andcreep-rupture ductility. This is particularly critical in sensitiveaircraft engine components where the superalloys find their greatestutility and particularly in the highly-alloyed superalloys, i.e., thosecontaining a large amount of the precipitated secondary phase. Sincedesign criteria must be established on the basis of the weakestcomponent in the system, the wide scatter band associated withconventionally processed superalloy components has demanded designswhich do not adequately utilize the mechanical properties of which thesealloys are capable.

SUMMARY OF THE INVENTION The present invention comprises a thermalprocessing technique for the as-worked, precipitation hardeningnickel-base superalloys, particularly the highly alloyed compositions ofthe 7-7 type. It contemplates, after working but before aging,subjecting the alloy to a duplex heat treatment comprising a first heattreatment estab lishing uniformity of the precipitated phase throughoutthe alloy microstructure and nucleation of a new grain structure underconditions of restricted growth due to the presence of a secondaryphase, and a second heat treatment providing uniform solutioning of thesecondary phase and controlled grain growth by relying upon grainannihilation under conditions of uniform strain energy distributionwithin the polycrystalline aggregate. The first heat treatment isperformed within the range of 25-100 F. below the true secondary phasesolvus and the solutioning heat treatment within about 25 F. of thesolvus. The overall result is a uniform, reproducible microstructurefrom which subsequent aging heat treatments can promote maximum alloystrength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic problem leading tothe generation of the present invention was the non-uniformity ofsuperalloy mechanical property response in large gas turbine enginedis-k forgings following normal deformation and heat treatment. Thesource of property scatter problem was eventually traced to extensivesegregation of the secondary phase and non-uniform recrystallization ofthe polycrystalline structure, the problem being particularly acute inthe highly alloyed compositions.

A thermal process to provide a uniformly controlled microstructuralcondition in the as-Worked, precipitationhardening nickel-basesuperalloys was developed. Basically, the process prescribes exposure ofthe alloy in the worked condition to a temperature 25 -100 F. below thetrue secondary phase solvus followed by a second exposure 025 F. belowthat temperature.

Any amount of work above approximately ten percent deformation willcause recrystallization. The present process effects recrystallizationat a low temperature where the presence of a secondary phase willinhibit grain growth followed by a treatment at about the solvustemperature with control of grain growth by annihilation of onerecrystallized grain by another rather than, as in conventionalprocessing, by annihilation of a deformed grain by a recrystallizedgrain, the latter being the faster and more coarsening operative.

The initial heat treatment causes partial dissolution of the secondaryphase in alloy segregated areas followed by elemental diffusion toregions of alloy depletion and subsequent reprecipitation to anequilibrium concentration. The net transfer established uniformity ofthe precipitated phase through the worked structure. Additionally, thisinitial exposure promotes partial or complete recrystallization andannealing, depending upon the specific energy input during working,under conditions of restricted grain growth.

The second heat treatment produces uniform solutioning of the secondaryphase and enables the completion of recrystallization and annealingwhile still maintaining a condition of inhibited grain growth. Theoverall result is a. uniform structure from which subsequent aging heattreatments can promote maximum alloy strength.

The time required for the initial heat treatment is essentially onewhich, based on metal diffusion rates, is equivalent to the timerequired for equilibration at the true solvus temperature. The timerequired for the second 3 heat treatment is that for which equilibriumbetween the primary and secondary phases can be achieved.

After the normal Working is completed, the true solvus of the componentis determined experimentally. At this temperature for the nickel-basesuperalloys of the -q" type it has been determined that a ten hourexposure will equilibrate the entire system, other than that of grain 4are employed and the part is usually air cooled (although this is of noimportance because of the metallurgical phenomena involved).

The solution heat treatment is accomplished at about the true solvustemperature, usually specifying heat treatment 25 F. below the solvus tocompensate primarily for thermal variations within commercial furnaces.

ROOM TEMPERATURE TENSILE PROPERTIES Elongation Reduction of area 0.2%yield (K 5.1.) Ultimate (K s.i.) (percent) (percent) Standard Duplexprocessing processing Standard Duplex Standard Duplex Standard Duplex151. 9 144. 0 216. 0 202.8 18. 19.0 20.4 26. 0 146. 6 144. 0 208.0 201.2 16. 5 21.0 19.2 27.0 152. 0 144. 0 215. 2 200. 0 16. 0 20. 0 17. 4 23.0 149. 0 140. 0 214. 0 195. 6 16. 5 20.0 19.6 27.0 152. 0 146. 6 217. 0206. 0 17. 5 21. o 18.1 29. 0 152. s 141.0 209. 5 196. s 12. 0 21. 013.0 27. 0 155. 2 144. 0 215. 0 19s. 0 13. 0 20. 0 13. a 22. 0 160.8149. s 222. 0 210. 4 13. 0 21.0 12. 3 23. 0

159.8 150. 9 217. 5 212. 1 1 1. g 21.0 11 .5 29. 0 L LQ 144. 0 181. 7202. 0 y 20. 0 3 27. 0 Elli 145. s 214. 0 204. 0 12.0 19. 0 14. 2 24. 0160. 7 144. 0 220. 0 197. s 12. 5 20. 0 18. 1 25. 9 165. s 136. 0 225. 5195. 6 10. 0 20. 0 11. 9 22. 3

1,400 F. TENSILE PROPERTIES 141. 3 130. 4 s. 6 156. 5 25. 0 33. 0 39. 444. 0 13s. 4 132. 4 178.0 152. 4 21. 5 32. 0 2s. 6 42. 0 119. a 131. 2159. 0 154. 2 24. 0 33. 0 42. 4 44. 0

128. 0 12s. 4 150. 4 152. 4 26. 5 31. o 38.2 51. 3 132. 2 160. 29. 5 43.6 135. 0 160. a 25.5 42.0 141.7 167. 5 30.0 45. 3 143 163. 5 15. 0 19. e132. 5 162.1 23.0 30. 0 29. a 15s. 7 22. 0 37. 6

1 Property sought.

Norm-Underlined values failed specification.

boundary area reduction which is a reduction of crystal STRESS RUPTUREPROPERTIES surface energy. With knowledge of the true solvus and the tenhour baseline as the time for certain equilibration, the Life 11114.)time requirements to achieve the same conditions at S 1 est St St d Stad lower temperatures may be found from the following $3: $4. 1; gff, iDuplex 2 d Duplex u on: eq Smo0th 1,400 85.0 2 77.4 17.1 19.3

1) 2 2 7.1 74.4 19.1 24.7 log t T 1 3 77.9 29.7 28.8 1 140. 6 20. 9 19.1 where 27. 9 70. 3 9g 25. a

2.4 2. T =true solvus 3 3 Z in Z T =lower temperature being considered 11 0 1 s5. 0 136, 5 1 1() 0 K==con n Sta t 0.1% CREEP PROPERTIES Theconstant K for the nickel-base superalloycomposr- Time to L0% (hm) tionsprec1p1tat1ng the 7 phase remalns essentially constant at a valueapproximating 24 for all of these alloys. standard Dumex Once thetime-temperature relationships are established, 1,300 74. 0 144,; 168. 5the practical aspects of heat treatment become controlling 173. 4 242. 0insofar as temperature conditions are concerned. In the m initial heattreatment at temperatures more than 100 F. 125; 223") below the solvus,heat treatment times become extremely 5L3 174"] long, thus establishinga practical lower temperature limit. 139-18 As the temperature is raisedapproaching the solvus, the 13-011 1,300 74.0 1 150.0 problemsassociated with conventional processmg are approached. Thus, an initialheat treatment 25 "-100 1 Specification property. below the truesecondary phase solvus is established. No'rE.Underlined valuesfailedspecification. Within this range closer control may be desired withspecific temperature selection based primarily upon the fundamentalproblem to be solved or the condition to be The preferred duplexprocessing for a number of 1 achieved dunng the cycle. If more uniformrecrystalliza- Ioys is as follows.

tion is demanded, a lower temperature is used. If alloy segregation isthe main concern, a higher temperature is employed.

In practice once the processing parameters have been established,otherwise standard heat treatment techniques 5 Alloy description: Heattreatment Astroloy (forged) 2025 F. for 50 hours, air cool, plus 2065 F.for 4 hours. Waspaloy (forged) 1825 F. for 50 hours, air cool, plus 2150F. for 4 hours. Temperature control *-l F.

Comparative test data for Astroloy showing mechanical propertyuniformity and reduction of scatter is summarized below.

Although the invention has been described in detail and with referenceto several preferred embodiments for the purposes of illustration, theinvention in its broader aspects is not limited to the exact detailsdescribed, for obvious modications will occur to those skilled in theart.

What is claimed is: 1. The method of processing the precipitationhardening nickel-base superalloys of the 'y,'y'-type containing asessential elements about, by weight, 5-30 percent chromium, up to 0.2percent carbon, and at least 4 percent of at least one element selectedfrom the group consisting of aluminum and titanium, which comprises,after working but prior to aging:

heat treating the as-worked superalloy at a temperature about 25 -100 F.below the true 7 solvus temperature to provide a uniform distribution ofthe 7' phase;

and subsequently heat treating the superalloy at about the true 7' phasesolvus temperature to effect uniform solutioning and essentiallycomplete recrystallization with inhibited grain growth.

2. The method according to claim 1 wherein: the latter heat treatment isconducted within 25 F. of but below the true 7' phase solvustemperature. 3. The method of processing the precipitation hardeningnickel-base superalloys of the '-type containing as essential elementsabout, by weight, 5-30 percent chromium, up to 0.2 percent carbon, andat least 4 percent of at least one element selected from the groupconsisting of aluminum and titanium, which comprises, after working:

heat treating the as-worked superalloy at a temperature 25-100 F. belowthe true 'y' phase solvus temperature for a minimum of about 10 hours;subsequently heat treating the alloy within about 25 F. of but notsubstantially exceeding the true 7' phase solvus temperature for aminimum of about 4 hours; and then aging the superalloy to promote alloystrength.

References Cited UNITED STATES PATENTS 2,497,667 2/ 1950 Gresham et a1148-162 X 3,272,666 9/1966 Symonds 148-162 X 3,048,485 8/1962 Bieber148- 162 X 3,145,124 8/1964 Hignett et a1. 148-162 3,147,155 9/1964 Lamb148-115 R 3,576,681 4/1971 Barker et al -171 X CHARLES N. LOVELL,Primary Examiner US. Cl. X.R.

